CBSE - XII Science - Biology - Principles of Inheritance and Variation
Linkage and crossing over of genes are alternatives of each other. Justify with a suitable example.
Linkage is the tendency of certain loci or alleles (genes) to be inherited together while, crossing over is the segregation of genes, e.g. the genes on the chromosome either follow linkage path or crossover, to form the gametes during gametogenesis in human.
There is some linkage between all the genes located on the same chromosome. The linkage strength depends on the percentage of the distance between the two. However, linkage can be easily broken by crossing over. When genes are located on the same chromosome, then there is possibility of two situations, either a crossing over between the two genes or no crossing between two genes. Crossing over always occurs if genes are located very far from each other – 50% recombinants, 50% parental.
Example: In Drosophila, a yellow bodied white eyed female was crossed with brown bodied red eyed male. The F1 progeny produced was intercrossed to obtain the F2 progeny. The F2 phenotypic ratio of Drosophila deviated significantly from Mendel’s 9:3:3:1. He found that the genes for white and yellow were very lightly linked and showed only 13% recombinants while white and miniature wing showed 37.2% recombination. Morgan reasoned that the genes for eye colour and body colour are closely located on the ‘X’ chromosome. They show linkage and therefore, are inherited together. Recombinants were formed due to crossing over but at low percentage.
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